Digital Circuit 03 OR-gate

Explaination how an OR-gate works in the usual cmos technology.
The OR-gate is one of the three fundamental elements of digital electronics, together with the AND-gate and the inverter. It consists of three NMOS transistors and three PMOS transistors. One pair of NMOS and PMOS form an inverter, the rest forms an NOR-Gate. The output of the NOR is inverted to OR. That is why most integrated chips that form an OR, also have an output for the NOR: it simply does not need any more transistors, because the NOR is already created. So you need only one pin to carry out the NOR.

an OR-gate has two inputs and one output.

Only when both inputs are low, then both NMOS transistors block the path between the ground node and the red node and both PMOS transistors connect, so that the red node is connected to the positive source. So the red node is high and the output is low

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The OR-gate
The OR-gate

Just like the AND-gate we learned before, an OR-gate consists of three NMOS transistors plus three PMOS transistors.

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Also the pair of transistors between the red node and the green output is an inverter, just like the inverter in the AND-gate. But here the other four transistors are wired differently than in the AND-gate: Here in an OR-gate the PMOS transistors between the red node and the power supply node are put in series, and the NMOS transistors between the red node and ground are placed paralell. (At the AND-gate it was vice versa)

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Title=Digital Circuit 03 OR-gate

Author=Benedikt Sessler

Date=25.Sept 2020